Notes

Cleavage is a series of rapid mitotic divisions of the zygote which convert the single celled zygote into a multicellular structure called blastula (blastocyst). Process:
About thirty hours after fertilization, the newly formed zygote divides into two cells, the blastomeres, in the upper portion of the Fallopian tube.
This is the first cleavage. The next division occurs within forty hours after fertilization. The third division occurs about three days after fertilization. During these early cleavages, the young embryo is slowly moving down the Fallopian tube towards the uterus.
At the end of fourth day, the embryo reaches the uterus. It has 8-16 blastomeres and this solid mass of cells is known as morula (little mulberry) as it looks like a mulberry. When the blastomeres divide completely the cleavage is called holoblastic.
Significance of Cleavage:
Cleavage brings about (i) the distribution of the cytoplasm of the zygote, amongst the blastomeres, (ii) increased mobility of the protoplasm, which facilitates morphogenetic movements necessary for cell differentiation, germ layer formation and the formation of tissue and organs, (iii) the restoration of the cell size and the nucleo- cytoplasmic ratio characteristic of the species, (iv) Unicellular zygote is converted into multicellular embryo.